In the Enhanced General Packet Radio System (EGPRS) the length of checksum (CRC, parity bits, 3GPP 05.03) for RLC packets in the Layer 1 interface is 12 bits, while in Release 1997 (R97) and R99 of the General Packet Radio System (GPRS) the checksum is 16 bits. Layer 1 uses the checksum for detecting whether a received packet was corrupted in the air interface. It should be noted that while the data lengths in EGPRS are longer than in GPRS, the CRC is shorter. As a result, it is much more probable in EGPRS that a RLC packet can be corrupted during transit in the air interface in such a manner that its CRC is still correct. In this situation the Layer 1 sees the frame as being correctly received and delivers it to the RLC layer. There are at least two problems that can arise when undetected packet corruption takes place.
The first problem occurs if the first data bit (FBI) was not corrupted, and is as follows. After receiving all of the frames belonging to a Logical Link Control frame (LLC, the data protocol layer above the RLC), the RLC delivers the Packet Data Unit (PDU) to the LLC. The LLC performs its own checksum computation and detects an error. In this case the LLC packet is discarded.
The second problem occurs if the first data bit (FBI) is changed from a 0 to 1, and is as follows. The mobile station erroneously detects that the block is the last block in the downlink (DL) direction and ends the currently established Temporary Block Flow (TBF). The network then must again establish the TBF. This type of error is expected to occur less frequently than LLC checksum errors.
In either case, the user perceives the occurrence of these failure modes as a reduction in data throughput.
Since packet corruption happens more frequently under poor radio conditions, the link adaptation procedures performed by the wireless network could alleviate the problem. However, if the performance of the EGPRS CRC error detection was made similar to that of GPRS R97, the length of the CRC in EGPRS would need to be longer than 16 bits. This is due at least in part to the fact there are more bytes in EGPRS blocks than in GPRS blocks, and that data blocks modulated with EGPRS 8PSK modulation are more susceptible to errors than are GPRS GMSK modulated data blocks.
As may be appreciated, increasing the number of CRC bits in EGPRS is not readily accomplished, as it would require a substantial standardization change, with corresponding changes having to be made in the installed base of equipment (mobile stations, base transceiver stations, base station controller, etc.) This is clearly an undesirable solution.